flac: Use configure flags instead of *FLAGS variables.

For some reason, the flac configure script adds `-L/lib/` to `OGG_LIBS` if the `--with-ogg-libraries` flag is not given on the command line. This causes the build system to try to link some binaries to `/lib/libm.so`, which won't work since we're cross-compiling.

Currently, flac gets pointed at libogg using special `CPPFLAGS` and `LDFLAGS`. This patch changes the configure call to use `--with-ogg-includes` and `--with-ogg-libraries`, which fixes the failure and is a bit cleaner.

Oddly, I only encountered this problem when building for ARM.